> Can a provider forbid their customer from announcing allocated
> networks to another provider?
Yes. This is the discretion of the provider. Few forbid it completely,
but it does happen. More likely you'll see a list of conditions that
must be met before this is allowed, perhaps even including a small
additional fee to cover the added administrative headaches.
> Assume the following situation:
> ISP A gets customer C to sign an agreement for service. ISP A
> allocates network A.B.C.0/20 to customer C. A.B.C.0/20 is a
> subnet of CIDR block A.B.0.0/16, allocated to ISP A by our friends
> at the InterNIC as a non-portable address space.
> So, customer C goes to ISP B, and says, I'd like to buy service
> from you, and announce network A.B.C.0/20 to you.
I would think that ISP A would be all for it, since as soon as ISP B
starts announcing the /20, all in-bound traffic will come through ISP B
and relieve some of ISP A's bandwidth. Since ISP A is announcing a /16,
the /20 announced by ISP B would take precedence, being a more specific
Why exactly do you think this makes ISP A happy? Most likely the link
to ISP B is a link that ISP A would have preferred to sell themselves.
And to "relieve some of ISP A's bandwidth" doesn't make ISP A any happier
since they have to engineer for that bandwidth anyway for those cases
where ISP B fails.
Ok, I'll allow that some ISPs get charged by the packet by their upstream
provider and thus would have slightly lower overall costs as ISP A under
this scenario. I'd be surprised if that difference offsets added costs
elsewhere in the average case.
I would think that ISP A would not have much to stand on from a legal
- your contract does not prevent you from buying service from another
provider (I'm assuming this),
Probably true, but not necessarily so. There's nothing illegal about
an exclusive contract.
and (I further assume) it does not
prohibit you from letting another ISP announce those routes
This is a stretch. Chances are that your contract does not specify
anything about announcing routes to third parties, but since the larger
block was initially assigned to ISP A, it reasonably follows that ISP A
can regulate those blocks through separate policy so long as your
contract does not say otherwise. Even so, many (most?) contracts these
days make sure by proclaiming that addresses assigned by the ISP are
not the property of the customer and that the ISP retains some level
- as it stands now (and will until ARIN changes it), ISP A does not
actually *own* an IP addresses, and there is at least one RFC that
makes this pretty clear, as well as several InterNIC policies
This is true, but neither do you the customer.
- neither you nor ISP B are prohibiting ISP A from fulfilling their
obligations to you or the rest of your customers by announcing the
- the announcement of the /20 does not cost ISP A anything extra, nor
does it increase any contractual requirements for ISP A
In an ideal, well-informed world, these latter two would be true.
But they're not. The number of possible problems with external
routes multiplies with multiple announcements. Then, when those
problems do occur, ISP A and ISP B must both work at fixing things.
If ISP B is cooperative, coordination still requires extra time and
effort. If ISP B is not cooperative, ISP A either spends much more
time trying to troubleshoot ISP B's part, or ISP A gets a black eye
when ISP B screws up, or both. As more ISPs learn this, more are
writing provisions for it into their contracts.
This is a pretty gray area, though, but I think you have a pretty good
chance to be able to contest what ISP A says. That probably won't make
them go out of their way for you in the future, though.
That may or may not be true. An professionally-run ISP is happy to
explain the logic behind their policies and work with you to find an
adequate solution to your requirements. Questioning their initial
ideas might yield better solutions, but forcing your solution over
their objections is likely to yield a solution that doesn't fit your
ISP's architecture and thus doesn't serve you well. And that WILL
make them annoyed with you as well.